Nonwoven barrier fabrics have been developed which impede the passage of bacteria and other contaminants and which are used for disposable medical articles, such as surgical drapes, disposable gowns and the like. For example, such barrier fabrics can be formed by sandwiching an inner fibrous web of thermoplastic meltblown microfibers between two outer nonwoven webs of substantially continuous thermoplastic spunbonded filaments. The fibrous meltblown web provides a barrier to bacteria or other contaminants, while the outer spunbonded layers provide good strength and abrasion resistance to the composite nonwoven fabric. Examples of such fabrics are described in U.S. Pat. No. 4,041,203 and U.S. Pat. No. 4,863,785.
In the manufacture of this type of fabric, the respective nonwoven layers are thermally bonded together to form a unitary composite fabric. Typically, the thermal bonding involves passing the nonwoven layers through a heated patterned calender and partially melting the inner meltblown layer in discrete areas to form fusion bonds which hold the nonwoven layers of the composite together. Without sufficient melting and fusion of the meltblown layer, the composite fabric will have poor inter-ply adhesion. However, unless the thermal bonding conditions are accurately controlled, the possibility exists that the thermal bond areas may be heating excessively, causing "pinholes" which can compromise or destroy the barrier properties of the inner meltblown layer. Thus in practice, the thermal bonding conditions which are used represent a compromise between the required inter-ply adhesion strength on the one hand, and the required barrier properties which must be provided by the meltblown layer on the other.
The conventional spunbond-meltblown-spunbond type barrier fabrics also have limitations in the types of sterilization procedures which can be used. For some applications, it is desired that the fabric or garment be sterilized in the final stages of manufacture by exposure to gamma radiation. For example, the fabric or garment may first be sealed in a protective package, and then exposed to gamma radiation to sterilize the package and its contents. However, sterilization by gamma irradiation has been found to be unsuitable for many of the known medical barrier fabrics. Some of the polymers conventionally used in such medical barrier fabrics, such as conventional grades of polypropylene for example, are especially sensitive to degradation by gamma irradiation. Fabrics produced from such polymers tend to lose strength over time, becoming brittle as a result of the gamma irradiation. Also, the instability of the polymers to the irradiation results in the generation of distasteful odors in the product which are unacceptable to the consumer.
Conventional spunbond-meltblown-spunbond type barrier fabrics have limitations in the way they can be fabricated into a product, such as surgical gowns, surgical drapes, and the like. Typically these type of fabrics do not lend themselves to forming seams in a fabric construction by thermal bonding or welding. Further, such seams can be weak, and lack the integrity needed to provide a complete barrier to the passage of contaminants. Fabrics formed of conventional spunbond-meltblown-spunbond fabrics can be constructed by sewing, but this can be disadvantageous, since punching the fabric with a needle results in holes in the fabric, which impairs the integrity of the fabric and the continuity of the barrier properties thereof.
Various attempts have been made to overcome these limitations. For example, efforts have been made to render the polypropylene polymers more stable to gamma irradiation, such as by incorporating certain additives in the polymer to reduce the amount of degradation. For example, U.S. Pat. No. 4,822,666 describes a radiation stabilized polypropylene fabric in which a long-chain aliphatic ester is added to the polymer. U.S. Pat. No. 5,041,483 discloses incorporating a rosin ester into the polypropylene to stabilize the polymer and reduce the tendency toward odor generation after gamma irradiation. However, the use of such additives adds expense to the manufacturing process. Further, polypropylene is difficult to render gamma-stable at standard commercial dosage levels, even with the use of additives or stabilizers.
The component layers of spunbond-meltblown-spunbond type barrier fabrics can also be formed of polymers which are stable to gamma irradiation. Such polymers include polyamides, polyesters, some polyolefins, such as polyethylene, and the like. However, fabrics formed using high melt temperature polymers, such as polyamide and polyester, are not easily thermally bonded. The high temperatures which are required to sufficiently bond the fabric can destroy the meltblown barrier properties and the structure of the outer spunbonded webs. Adhesives can be used to bond the high melt temperature layers together, but this can result in stiffness of the resultant fabric and adds cost.
It would therefore be advantageous to provide a fabric that provides a barrier to the transmission of contaminants and which retains its strength and does not create an unpleasant odor when sterilized in the presence of gamma radiation. It would also be advantageous to provide such a fabric which exhibits good aesthetic properties, such as desirable softness, drape and breathability, as well as good strength and abrasion resistance, and which can be easily constructed into a product, such as a surgical gown.